Up to now, there has been proposed an image display apparatus which illuminates spatial light modulation elements utilizing polarization made of such as liquid crystal using an illuminator having a light source e.g. a discharge lamp, and projects an image of the spatial light modulation elements using a projection lens.
Such a projection-type image display apparatus has come into practical use, particularly as a large-sized image display apparatus. For example, in an image display apparatus which employs reflection-type liquid crystal elements having reflecting electrodes as spatial light modulation elements, the numerical aperture of the spatial light modulation elements can be enlarged to realize miniaturization of the configuration of the apparatus and high-definition image displaying.
The reflection-type spatial light modulation elements modulate polarization direction of incoming illumination light and reflect thus modulated light, according to an image to be displayed at respective pixels. Thus, when using reflection-type spatial light modulation elements, a polarizer which polarizes incoming light into the spatial light modulation elements and an analyzer which analyzes polarized components of predetermined direction alone from the light reflected by the spatial light modulation elements are necessary to be provided.
As the polarizer and analyzer, a polarization beam splitter (PBS) 101 can be used, as shown in FIG. 1. In the image display apparatus, illumination light emitted from a discharge lamp 102 outgoes from an illumination optical system 103 consisting of a parabolic mirror and a fly-eye lens, and comes into the polarization beam splitter 101 via a first condenser lens 104, a mirror 105 and a second condenser lens 106. The reflection plane of the polarization beam splitter 101, which is laid obliquely against the incoming illumination light, reflects S-polarized component alone of the incoming illumination light, and causes the S-polarized component to outgo to a dichroic prism 107 working as a color separation/composition element. Here, the polarization beam splitter 101 works as a polarizer.
The dichroic prism 107 separates the illumination light into R (red) light, G (green) light and B (blue) light, each of which comes into spatial light modulation elements 108, 109 and 110 corresponding to each color, respectively. The spatial light modulation elements 108, 109 and 110 modulate polarization directions of the separated color lights according to an image to be displayed. Then, thus modulated color lights are composited at the dichroic prism 107, and thus composited light returns to the polarization beam splitter 101. At this time, the polarization beam splitter 101 allows P-polarized component alone, toward the reflection plane, of the illumination light returned from the dichroic prism 107 to pass through the reflection plane, and causes the P-polarized component to outgo to a projection lens 111. Here, the polarization beam splitter 101 works as an analyzer for converting polarization modulation to intensity modulation.
The projection lens 111 projects the light of an image from the spatial light modulation elements 108, 109 and 110 onto a screen, not shown, to display an image.
In the image display apparatus described above, the reflection plane of the polarization beam splitter is made of dielectric multilayer films, which select polarized light in accordance with the difference of reflectance between P-polarized light and S-polarized light at respective interfaces of the dielectric multilayer films. Therefore, the reflection plane of the polarization beam splitter selects polarized light depending highly on wavelength and angle.
In the image display apparatus, when using bright illumination light of small F-number, the range of incident angle of illumination light toward the reflection plane of the polarization beam splitter becomes wide, which undesirably deteriorates the function of the polarization beam splitter as a polarizer. Namely, the image display apparatus employing the polarization beam splitter working as a polarizer as well as an analyzer cannot use bright illumination light, thus efficiency of light utilization in an illumination optical system can not be improved.
Also, separating color using a dichroic prism depends highly on polarization state. Namely, in the dichroic prism, the dichroic surface for separating color has different properties between for incoming light being S-polarized light and for outgoing light being P-polarized light. The polarization direction of the modulated light reflected by the spatial light modulation element is perpendicular to that of the incoming light, consequently, the efficiency of light utilization is deteriorated.
Further, in the case the dichroic surface is so constructed as to reflect R (red) light and B (blue) light of incoming S-polarized illumination light, G (green) light needs to pass through the dichroic surface. Generally, the dichotic surface has higher reflectance R (p) for S-polarized light as compared with reflectance R (s) for P-polarized light. Accordingly, actually, G (green) light component is also partially reflected to come into the spatial light modulation elements for R (red) light and B (blue) light, as shown in FIG. 2. The phenomenon described above consequently deteriorates color separation property and color reproduction property of an image to be displayed.
In case of using a light source of irregular emission spectrum distribution, illumination light of regular wavelength distribution may not be obtained. In this case, when the illumination light is separated into R (red) light, G (green) light and B (blue) light to be modulated and thus modulated lights are composited, desirable color reproduction range may not be obtained.